The gaze direction of a person may be used to select options displayed on a computer screen. Thus, an eye tracker device providing information on the gaze direction may be used, for example, as a pointing and selecting device instead of a computer mouse.
When light impinges on the eye, several reflections occur on the boundaries of the lens, cornea and retina. These reflections provide reflection spots known as the Purkinje images. The reflection from the outer corneal surface provides the first Purkinje image, also called as the glint. The orientation of the eye may be determined based on the position of the pupil with respect to the position of the first Purkinje image.
Patent application PCT/FI2006/050043 discloses an eye tracker device comprising a diffractive beam expander to provide two illuminating beams. The use of two Purkinje images makes the determination substantially independent of the distance between the eye and the tracker device. Said application discloses also that the eye tracker device may be used in combination with a virtual display unit, wherein said virtual display unit is arranged to display virtual images.